Electronic ballasts for gas discharge lamps are often classified into two groups according to how the lamps are ignited: (1) a preheat type ballast; and (2) an instant start type ballast. In preheat ballasts, the lamp filaments are preheated at a relatively high level (e.g., 7 volts peak) for a limited period of time (e.g., one second or less) before a moderately high voltage (e.g., 500 volts peak) is applied across the lamp in order to ignite the lamp. In instant start ballasts, the lamp filaments are not preheated, so a higher starting voltage (e.g., 1000 volts peak) is required in order to ignite the lamp. It is generally acknowledged that instant start operation offers certain advantages, such as the ability to ignite the lamp at a lower ambient temperatures and greater energy efficiency (i.e., light output per watt) due to no expenditure of power on filament heating during normal operation of the lamp. On the other hand, instant start operation usually results in considerably lower lamp life than preheat operation.
Because a significant amount of power can be unnecessarily expended heating the lamp filaments after the lamp is ignited, it is desirable to have preheat type ballasts in which filament power is minimized or eliminated once the lamp has ignited. One approach for preheating ballasts employs switching circuitry such as a filament control circuit that disconnects the source of filament power from each of the filaments after the lamp ignites. However, when such switching circuitry is used with ballasts driving multiple fluorescent lamps, there have been problems preheating and igniting lamps which have been disconnected from the ballast and then reconnected back to the ballast. One solution to ignite such reconnected lamps has been to cycle the power supplied to the ballast (i.e., turn the power off, and then back on).
In ballast circuits driving three (3) or more lamps, the outermost lamps are usually connected directly to the ballast circuit. Thus, disconnecting the outer lamps may cause an open circuit which can be detected. When an outer lamp is reconnected, it closes the circuit so that preheating and/or ignition can be initiated. However, the inner lamps, such as the middle lamp in a three lamp circuit, are connected with one or more of the outer lamps but are not directly connected to the ballast circuit. Hence, removing and reconnecting an inner lamp may not close an open circuit so that its reconnection is difficult to detect. Accordingly, re-igniting a disconnected and reconnected inner lamp has typically required cycling of the power. To avoid the need for cycling the ballast power when an inner lamp of a plurality of lamps connected to the ballast circuit is taken out and then reconnected to the circuit, there is a need for a ballast circuit that detects the reconnection of an inner lamp to preheat and/or ignite the reconnected lamp without requiring cycling of the power.